Coral reefs are early casualties of climate change, but not every coral reacts the same way to the stress of ocean warming. Now a Northwestern University research team is the first to provide a quantitative “global index” detailing which of the world’s coral species are most susceptible to coral bleaching and most likely to die.

The world currently is experiencing the longest global coral bleaching event ever recorded, with the Great Barrier Reef and U.S. reefs among those suffering. Bleaching happens when stressed corals expel their life-providing algae, turning coral reefs stark white as their skeletons show through. Some corals rebound, but many do not.

The coral bleaching response index was published today (April 13) as an Early View article by the journal Global Change Biology. Based on a massive amount of historical data, the index can be used to compare the bleaching responses of corals throughout the world and to predict which corals may be most affected by future bleaching events.

“Coral bleaching is an inescapable example of the effects of climate change,” said Timothy D. Swain, the study’s first author and a postdoctoral fellow at the McCormick School of Engineering. “We can see it with our eyes, and we also clearly see the progression of climate change in our data. Our goal is to use data to understand what is driving bleaching and learn how we can protect the world’s coral reefs, so we don’t lose them so quickly.”

Swain is a member of the interdisciplinary research team that analyzed publicly available data on nearly half the world’s corals -- including actual measurements of bleaching -- to produce the global index. The team was led by molecular biologist Luisa A. Marcelino and included Vadim Backman, both professors at McCormick.

The global index is a standardized measure of vulnerability, by species of coral, to thermal stress. It identifies the species most susceptible to bleaching and those most likely to perish as a result of the damage; hardier species also are identified. The index ranks the corals’ susceptibility to thermal stress from 1 to 100, with the most susceptible first in the list.

The index provides a valuable new tool to conservationists and park managers committed to preserving coral reefs and scientists interested in learning more about the hundreds of reef-building corals.